1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a semiconductor device wherein a silicon substrate of a high resistance is used to secure the Q value of passive elements and to restrict leakage current to the substrate so that noise between elements can be reduced, and a method for manufacturing the same.
2. Description of the Background Art
As mobile terminal apparatuses, such as cellular phones, have come into wide use, the manufacture of high frequency devices at a lower cost has become required. Therefore, research has been carried out concerning the replacement of MMICs (Monolithic Microwave Integrated Circuits), wherein the expensive compound semiconductor GaAs is used, with MMICs or Bi-CMOSs (Complementary Metal Oxide Semiconductor) that use silicon (Si) as a base.
Si into which n conductive type or p conductive type impurities are doped is used as a substrate in a Bi-CMOS wherein a conventional Si substrate is used. Such Si is known as having a large dielectric loss in comparison with GaAs (see, for example, Japanese Patent Laying-Open No. 9-74102). That is to say, in the case that an Si substrate is used, a dielectric loss occurs between the Si substrate and a collector epitaxial layer in passive elements, such as inductors or capacitors, in transmission lines or in pad portions. Therefore, it is difficult to form capacitors or inductors having Q values that are sufficiently large in high frequency regions by using an Si substrate.
There has been a proposal that the utilization of a wafer having a high electrical resistance is effective in order to lower the dielectric loss that occurs in the above described Si semiconductor element (Abstract of IEEE BCTM (1996) pp. 134-137: “0.8 μm BiCMOS Process with High Resistivity Substrate for L-Band Si-MMIC Applications”). In addition, the above described Japanese Patent Laying-Open No. 9-74102 discloses an example of a high frequency circuit wherein a high resistance substrate is used. As illustrated in these, when a high resistance substrate is used it becomes possible to raise the Q value of the above described passive elements and, therefore, this is advantageous to form a high frequency circuit.
In the case that a high resistance substrate, as described above, is used, however, the following problems arise.
(1) Due to the low impurity concentration in a high resistance substrate there is a possibility that the impurity concentration will fluctuate and the conductivity type will change (a1) in the case that the impurity amount included in the substrate fluctuates slightly in the manufacturing process of this high resistance substrate, (a2) in the case that this high resistance substrate is contaminated with a substance that becomes an impurity during the process for forming elements in the high resistance substrate or (a3) in the case that the temperature or the atmosphere of a heat treatment fluctuate slightly during the process for forming elements in the high resistance substrate. Therefore, there is a risk that leakage current will increase from a collector region to the substrate in an element due to unexpected fluctuation in impurities and, thereby, the yield of the elements is lowered.
(2) There are p type or n type regions directly beneath the elements in a low resistance substrate so that a shield effect due to these conductive regions can be expected. However, the amount of dopant included in the crystal is small in a high resistance substrate and, therefore, the effect of the shielding of electromagnetic waves by means of the substrate is lowered. Therefore, the spread of electric flux lines from the elements becomes great in the lateral direction and there is a problem that the influence of noise between elements will becomes large when active elements are integrated at a high density.